By improvement of CRT displays and CRT controllers for controlling them, high-performance displays exhibiting a 1000.times.1000 resolution and over 16 million displayable colors have recently appeared and are used in personal computers and workstations. On the other hand, laptop type personal computers, which employ a liquid crystal display or plasma display for the purpose of weight reduction and space saving have appeared. The problem here is color processing when a program written on the assumption of using a high-performance display is made to run on a laptop type personal computer. In this specification, the term "color" is used in a broad sense to include monochrome and gray scales.
At present, as regards displayable colors, so-called flat-panel displays such as liquid crystal and plasma displays are far from the level of CRT displays. However they are at a stage in which gray scale display with several levels is practically used. Accordingly, the ability to distinguish the large number of colors used in a program such as described above using a flat-panel display is a problem.
A typical conventional method is to use a lookup table for converting n-bit (e.g., 8-bit) color data to m-bit (e.g., 2-bit) color data having a smaller number of bits, the lookup table maps 2.sup.n colors to 2.sup.m colors. As another example, in IBM Technical Disclosure Bulletin, Vol. 29, No. 2, pp. 811-812, published in July 1986, a technique is disclosed in which a color enable register is used with a mapping scheme to display a character whose foreground and background colors are specified by an attribute byte. The character may then be displayed on a monochrome liquid crystal display.
In color mapping using a lookup table, different original colors may be mapped to the same color. However, text cannot be viewed if such mapping occurs in the foreground and background colors of the text. In the technique described in the above TDB, the foreground and background colors can be displayed distinguishably, but its application is limited to the monochrome display. In addition, when reverse video is utilized, the output of some displays would lose resolution.
The present invention provides a method and an apparatus for color conversion which clearly displays changes in the color data.